(seasonal) periods of warming and cooling near the siArface and tend to 

 reach extremes in their mixed-layer structure. Cold waters at low lati- 

 tudes tend to lose their identity because of surface heating. During all 

 seasons, however, the surface positions of warm and cold currents in the 

 North Atlantic are ordinarily discernahle from injection temperatures. 

 Because strong negative vertical temperature gradients exist in cold wa- 

 ters during the warm season, upwelling does not satisfactorily explain 

 the continued presence of relatively cold surface water in the open 

 ocean. It appears rather that relatively cool waters mark the surface 

 position of cold currents where advection occurs below the heated layer. 



IV, ISOTACH PATTERNS OF MEAN CURREMTS AND THEIR APPLICATION TO 

 TEMPERATURE ANALYSES 



In areas where data are sparse or unevenly distributed or in unfamil- 

 iar areas where flow patterns are unknown, surface temperatures covering 

 extended periods should be compared with mean isotach patterns. This pro- 

 cedure is especially helpful where cvirrents are apparently constrained by 

 topography; for example, in inland seas and near coastal areas. Studies 

 indicate that high temperatures are related to high isotach values and 

 deep water. An isotach analysis for the Caribbean Sea and the Gulf of 

 Mexico is shown in Figure 6. Isotach configurations were employed as 

 warm and cold flow patterns for interpretation of the temperatures on the 

 composite chart (Figure 7). A series of temperature charts based on the 

 configurations in Figure 6 show only minor pattern changes over a period 

 of several months. Figure 7 indicates an overall surface temperature 

 range of about 9° F even in these latitudes; temperature contrasts be- 

 tween adjacent waters are expected to be appreciably greater below the 

 depth of wave mixing. 



Although significant details of the circulation are lost by averag- 

 ing temperatures and drift values over one-degree quadrangles, Figiire 8, 

 in which February drift values (1935-^5) are compared with mean isotherms 

 for February I96I, shows that low temperattjres correspond to minimum 

 drift and vice versa. Of particular interest is evidence of counter cur- 

 rents south of the 32nd parallel. In connection with the temperature pro- 

 file and currents off the California coast (Figure 9), it will be noted 

 that, except at the extreme western end of the section where the warm axis 

 is slightly displaced to the right of a marked salinity minimum, currents 

 change direction near the n^ximal and minimal portions of the trace in 

 agreement with the relation discussed on page k. 



Since mean drift values are ordinarily computed for one-degree quad- 

 rangles over long tii&e intervals, only the more gross and permanent fea- 

 tures of the circulation are indicated by them. This may be especially 

 true in the deep ocean where current systems are not topographically con- 

 strained. Effectiveness of isotach analyses should be greatly enhanced 

 when data density permits averaging of drift values over much smaller 

 unit areas than the present one-degree quadrangle. For example, com- 

 putations of mean drift for a portion of a pesinanent warm tongue (Figure 

 la) are low, because drift values, although high along its boundary^are 

 oppositely directed. 



